The invention relates to an optical scanning device for scanning an information track of an optically readable record carrier, which scanning device includes a radiation source, a radiation-sensitive detection system, and an optical scanning unit for focusing a scanning beam produced by the radiation source to a scanning spot on the record carrier and for imaging the scanning spot on the detection system, the scanning unit including a lens system having an optical axis, which lens system comprises a first lens unit having a main lens and a second lens unit having an auxiliary lens, which scanning device further includes a first detection means involved in detection of the main lens with respect to the record carrier, and a second detection means involved in detection of the auxiliary lens, the scanning unit further including a first electrical drive unit for moving the first lens unit in dependence upon a signal supplied by the first detection means, and a second electrical drive unit for moving the second lens unit with respect to the main lens in dependence upon a signal supplied by the second detection means, the second drive unit comprising a first part and a second part which cooperates with the first part via an air gap, the first part being secured to the first lens unit and the second part being secured to the second lens unit.
Such a scanning device is known from the International Symposium on optical memory and optical data storage, conference edition Jul. 8-12, 1996 (ISOM 96); OFA 2-1/345-OFA 2-3/347; A. 0.8 Numerical Aperture Two Element Objective Lens for the Optical Disk; Kenji Yamamoto et al. The known device is intended for scanning a magneto-optical disc and includes a first holder with an objective lens and a transparent plate and a second holder with a solid immersion lens (SIL). The magneto-optical disc has been provided with an information layer and a transparent layer, the information layer being read via the transparent layer. For the focusing and tracking purposes a focus actuator and a tracking actuator have been provided, which actuators each include a moving-coil drive for driving the first holder. A SIL actuator has been provided for moving the second holder with the solid immersion lens in a focus direction in dependence upon a variation in capacitance of a capacitor formed between the two holders. The object of this is to preclude spherical aberration owing to deviations in thickness of the transparent layer. In this respect reference is also made to JP-A 08212579A. Apart from thickness variations an optical disc exhibits curvature, for example flexure owing to the fact that the disc is supported only centrally during scanning. As a result of this, the optical disc occupies a certain oblique position with respect to the scanning beam, which gives rise to coma and astigmatism during scanning. Such optical aberrations are not precluded by the known measures.